Prior to this invention, pipetting devices have been used for dispensing bacteria-containing suspensions into suitable receptacles such as the wells of a microtitration plate (or microtest plate, as it is also called). In addition, pin inoculators have been used for adding bacteria to a nutrient broth or other liquid suspension. In both cases the dispensing instrument or at least parts thereof become contaminated, thus requiring the contaminated parts to be either discarded or sterilized.
Devices of the foregoing types are used for dispensing contaminating materials in various tests. In an antimicrobial susceptability test, for example, a pipette is customarily used to pick up small quantities of a premixed suspension of bacteria in a nutrient broth and to dispense the picked up samples into separate microtest wells containing different concentrations of antibodies. Where used for this purpose, the pipette is required to be capable of reproducibly dispensing uniform volumes of liquid samples in microliter quantities to assure that equal amounts of bacteria are intermingled with the different concentrations of antibodies.
Pipettes which meet the foregoing requirements are customarily of the mechanical type which typically have a piston and cylinder assembly for applying a suction to pick up liquid samples. Pipettes of this type are expensive and therefore require reuse to justify their cost. Some pipetting devices of this type are equipped with disposable tips to avoid the inconvenience of sterilization provided that the device is carefully used to confine contamination to the liquid pick-up tips.
Instead of premixing the bacteria with the nutrient broth, an antimicrobial susceptability test may be carried out by using a reusable pipette type dispenser just to dispense a bacteria free broth into the microtest wells and by thereafter using a disposable pin inoculator to transfer the bacteria directly from a culture to the microtest wells. Use of disposable pin inoculators eliminates the contamination problem, but gives rise to two significant drawbacks.
First, the amount or number of bacteria picked up with a pin inoculator depends to a significant extent upon operator technique, making it somewhat difficult to repeatedly pick up equal, preselected amounts of bacteria. Second, a greater number of steps or operations are involved as compared with the procedure where the bacteria are premixed with the nutrient broth.
Accordingly, pipette type dispensers and pin inoculators both have drawbacks when used for the foregoing purposes. Furthermore, known pipetting devices of the convenient hand-held type are not capable of dispensing 96 liquid samples without refill to accommodate a standard 96-well microtitration plate. Some pipette type dispensers are equipped with 96 channels or tips to load all 96 wells without refill, but they are of the relatively expensive bench type such as the one described in U.S. Pat. No. 3,568,735, which issued to J. F. Lancaster on Mar. 9, 1971.